ON REMELTING OF CAST IRON.
From trials conducted by Ledebur, it appears that cast iron is rendered suitable for foundry purposes—i.e., to fill the moulds well and to yield sharp and definite forms free of flaws, to be cut with a chisel, and turned on a lathe—through a certain percentage of graphite, whose presence depends on that of carbon and silicium. Cast iron free of silicium yields on cooling the entire amount of carbon in the amorphous state, while presence of the former metal gives rise to the formation of graphite, and, consequently, causes a partial separation of carbon. Iron suffers on casting loss of graphite, assumes a finely-grained texture, becomes hard and brittle, and is changed from gray to white. In view of the fact that samples of cast iron with equal percentage of silicium and carbon yield on casting a different product, it has become necessary to institute experiments as to the cause of this behavior. Samples of cast iron were therefore repeatedly melted, and thin sections of each melt examined; these sections exhibited a gray color, though less apparent than in the unmelted sample, and possessed sufficient softness to admit boring and filing. During these processes of fusing, the amount of silicium, carbon, and manganese had been gradually decreased, and amounted to 12.7, 17.6, and 24.4 per centum for silicium in the three samples examined. It also was observed that the more manganese the iron contains the less readily the percentage of silicium is diminished; and since manganese is more subject to oxidation than silicium, it is capable to reduce silicic acid of the slag or lining to metal, and thus to augment the amount of silicium in cast iron. The percentage of carbon also suffers diminution by oxidation, which latter process is impeded by presence of manganese, a fact of some importance in melting of cast iron in the cupola furnace. An excess of manganese renders cast iron hard and brittle, and imparts to it the properties to absorb gases, while an amount of 1.5 per centum, as found in Scotch iron, undoubtedly has the effect to produce those properties for which this iron is held in high repute. The amount of copper is not visibly altered by fusion, but that of phosphorus and sulphur slowly increased.
Experiments in regard to the relation between chemical composition and strength of the material have established that a large amount of silicium, graphite, manganese, and combined carbon reduce the elasticity, strength, and tenacity of cast iron, and that a limited percentage of silicium counteracts the injurious influence produced by an excess of combined carbon. On remelting of cast iron, increase in tensile strength was observed, which attained its maximum in iron with a small percentage of silicium after the third, and in such with a large amount after the fourth melting. The increase in tensile strength was accompanied by a loss of silicium, graphite, and manganese coupled with a simultaneous augmentation of combined carbon. A fifth melting of the cast iron renders it hard, brittle, and white, through oxidation of silicium and subsequent lowering of the amount of carbon. On lessening the percentage of combined carbon with formation of graphite the injurious influence of the accessorial constituents of cast iron is diminished, especially that produced by the presence of phosphorus.—Eisenhuettentechnik.
FEEDING BOILERS AT THE BOTTOM.
One of the most important things to be considered in boiler construction is the position and arrangement of the feed apparatus, but it is, unfortunately, one of the elements that is most often overlooked, or, if considered at all, only in a very superficial manner. Many seem to think that it is only necessary to have a hole somewhere in the boiler—no matter what part—through which water may be pumped, and we have all that is desired. This is a very grave error. Many boilers have been ruined, and (we make the assertion with the confidence born of long experience) a large number of destructive explosions have been directly caused by introducing the feed water into boilers at the wrong point.
On the location and construction of the feed depends to some extent the economical working of a boiler, and, to a great extent, especially with certain types of boilers, its safety, durability, and freedom from a variety of defects, such as leaky seams, fractured plates, and others of a similar kind. And it is unfortunately true that the type of boiler which from its nature is most severely affected by mal-construction, such as we are now speaking of, is the very one which is the oftenest subject to it. We are speaking now more particularly of the plain cylinder boiler, of which there are many in use throughout the country.
Plain cylinder boilers are, as a rule, provided with mud drums located near the back end. As a rule, also, these boilers are set in pairs over a single furnace, and the mud drum extends across beneath, and is connected to both, and one end projects through the setting wall at the side. Our illustrations show a typical arrangement of this kind. Fig. 1 shows a transverse section of the boilers and setting, while Fig. 2 shows a longitudinal section of the same. It is a favorite method to connect the feed pipe, F, to the end of the mud drum which projects through the wall, and here the feed water is introduced, whether hot or cold; and there is really not so much difference after all between the two, for no matter how effective a heater may be, the temperature to which it can raise water passing through is quite low compared with the temperature of the water in the boiler due to a steam pressure of say eighty pounds per square inch. The difference in the effect produced by feeding hot or cold water at the wrong place is one of degree, not of kind.